NIH-funded researchers at the University of Pennsylvania request funds for a shared instrumentation grant to replace an 18-year old scanning electron microscope (SEM) that has been a workhorse for the School of Medicine (SOM). Despite the best efforts and significant investment of both our faculty and staff and the microscope company, this only SEM available to Penn Medicine researchers has been unreliable for the past few years and has recently become practically unusable. The aim of this proposal is to provide a FEI Quanta 250 microscope that will serve the basic needs for the examination of traditional dehydrated, metal coated samples with SEM, but also including the capability for correlative light microscopy-SEM, imaging of hydrated samples, and back-scattered electron detection of colloidal gold particles. This instrument will support a wide range of research throughout the SOM and across the campus. The studies described here require the substantially enhanced imaging capabilities of this system for NIH-funded research projects. Specifically, all investigators use various types of light microscopy in their research and require the ability to correlate the images obtained in this manner with higher resolution SEM images. Most of the scientists also require the identification and localization of specific macromolecules on the surface of cells or other structures, which requires a back-scattered electron detector. Because of the well-known artifacts of dehydration and metal coating, the ability to image hydrated specimens by environmental SEM is now necessary. We expect that demand and usage for the SEM will continue to rise as a consequence of these new capabilities. An indication of the very strong and broad support for this SEM is the commitment of substantial matching funds from the Department of Cell and Developmental Biology, together with the inclusion of the instrument in the Electron Microscopy Resource Laboratory, with core support and administration by the Department of Biochemistry and Biophysics, and the recognition and support of the SOM administration that SEM is a strategic necessity. This new microscope will allow fundamental biomedical problems to be addressed that would otherwise not be possible, will ensure continued productivity from the researchers, and will maintain the microscopy-training mission of our core facilities.